I now want the content to be behind the background image and the rest of the template (title, page number...). The template will then have a cut-out with the content visible underneath. The background image can have .png or .pdf transparency.

nice idea. I tried but unfortunately this overwrites the title. Including it before the title let's it remain under the content. So I guess my question includes: how do I make the title come into foreground?
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Matthias123Apr 4 '14 at 17:57

mmm, that is too bad. Could you share an example of your code? It would be easier to modify an existing code in order to find an solution instead of having to write own stuff ...
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samcarterApr 4 '14 at 18:02

How about leaving the traditional title layer empty and including the title in the footline layer [after the passpartout]?
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samcarterApr 4 '14 at 18:06